Variable displacement hydraulic pump/motor with hydrostatic valve plate

ABSTRACT

A variable displacement hydraulic axial piston machine includes (a) a housing having an outlet opening; (b) a barrel mounted in the housing for rotation about a rotation axis; (c) a plurality of pistons movable in respective bores in the barrel; (d) a valve plate coupled to the barrel and having a fluid passage therein that ends at an outlet opening; and (e) a flow tube connecting the outlet opening in the valve plate to the outlet opening in the housing. The barrel and the valve plate are pivotally movable together about a pivot axis transverse the rotation axis for varying the displacement of the pistons in the bores. An end of the flow tube connects to the valve plate while allowing for axial movement of the flow tube relative to the valve plate in a direction parallel to the rotational axis of the barrel.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/369,209 filed Jul. 30, 2010, which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a hydraulic axial piston machine and, more particularly, to a variable displacement hydraulic pump/motor with a hydrostatic valve plate.

BACKGROUND OF THE INVENTION

A hydraulic axial piston machine has a barrel rotatably mounted within a pump housing. The barrel includes a plurality of circumferentially equally-spaced bores in which pistons reciprocate. Each piston bore has a port in the end of the barrel that lies against a valve plate that defines delivery and exhaust ports. As the barrel rotates, each piston bore port sequentially traverses the delivery and exhaust ports in the valve plate. In a pump mode, as each piston bore port traverses the delivery port low pressure fluid is drawn into the piston bore. When the piston bore port traverses the exhaust port, fluid is expelled at an increased pressure.

In axial piston machines with variable displacement the cylinder barrel also can be mounted for pivoting movements relative to a drive disk by means of either a slide bearing in the form of a valve plate which is slidably movable in a curved guide path in the machine casing, or by means of a yoke. While the valve plate design generally is compact, simple, and hydrostatically supports the load imparted on the machine casing, valve plate designs generally provide limited maximum total displacement angles due to a reduction in flow area as the valve plate is rotated. This is particularly disadvantageous for applications having reversing flow, i.e., applications where the machine is used as both a pump and a motor. In contrast, yoke designs allow for ease of porting even through large displacement angle ranges, but tend to be larger and more complex than valve plate designs. Further, yoke designs often are complex to manufacture and result in the need for extremely accurate manufacturing tolerances amongst various parts. See U.S. Pat. No. 4,991,492, for example.

SUMMARY OF THE INVENTION

The present invention provides an improved design of variable displacement hydraulic axial piston machine with advantages of both the hydrostatic sliding valve plate and yoke concepts in a single design, while eliminating or minimizing negative traits of both. The sliding valve plate in this design facilitates the creation of a hydrostatic pad between the sliding valve plate and the casing to reduce potential wear and reduce sliding friction. The sliding valve plate also has an operable running surface for engaging the piston barrel that is easier to machine flat. And finally, unlike the separate components in previous designs the sliding valve plate integrates the support functions of a valve plate and valve plate timing into a single unit. The machine provided by the invention also includes a separate flow tube for directing high pressure fluid to and/or from the sliding valve plate. The flow tube provided by the invention can translate axially as the sliding valve plate pivots about a pivot axis when the displacement changes, thereby reducing the tolerance requirements. Another advantage provided by the flow tube is that it allows for the connection of a feedback sensor directly on the pivot axis to more accurately monitor the angular position of the sliding valve plate about the pivot axis.

More particularly, the invention as set forth in the claims includes one or more of the features set forth in the following clauses.

A. A variable displacement hydraulic axial piston machine, comprising a housing having an outlet opening; a rotatable member rotatably-mounted in the housing for rotation about an axis; a valve plate having a fluid passage therein that ends at an outlet opening, a barrel mounted between the valve plate and the rotatable member, the barrel being coupled to the valve plate for rotation relative to the valve plate and the barrel being coupled to the rotatable member for rotation about a rotation axis, the barrel including a plurality of piston bores; a plurality of pistons movable in respective bores in the barrel and coupled to the rotatable member for rotation with the rotatable member, the barrel and the valve plate being pivotally movable together about a pivot axis transverse the rotation axis for varying the displacement of the pistons in the bores; a flow tube connecting the outlet opening of the valve plate to the outlet opening in the housing, the flow tube having a first connector at a first end that connects the flow tube to the valve plate while allowing for axial movement of the flow tube relative to the valve plate in a direction parallel to the rotational axis of the barrel, and a second end opposite the first end, the flow tube having a second connector at the second end that connects the flow tube to the outlet opening in the housing while allowing for angular movement of the flow tube about the pivot axis.

B. A machine as set forth in clause A or any other clause depending from clause A, where the first connector includes an end section of the flow tube that is received in a bore in the valve plate with a slip fit.

C. A machine as set forth in clause A or any other clause depending from clause A, where the second end of the flow tube is parallel to the pivot axis.

D. A machine as set forth in clause A or any other clause depending from clause A, where the barrel includes a plurality of circumferentially-spaced cylindrical bores.

E. A machine as set forth in clause A or any other clause depending from clause A, where the valve plate is movable relative to the housing.

F. A machine as set forth in clause A or any other clause depending from clause A, where the flow tube is formed as a separate structure from the valve plate.

G. A machine as set forth in clause A or any other clause depending from clause A, where the first end of the flow tube is movable axially within the bore of the valve plate.

H. A machine as set forth in clause A or any other clause depending from clause A, where the second end of the flow tube is received in a bore that is coaxial with the pivot axis.

I. A machine as set forth in clause A or any other clause depending from clause A, where the second end of the flow tube is received in a bore with a slip fit for rotation about the pivot axis.

J. A machine as set forth in clause A or any other clause depending from clause A, wherein a radial opening extends through the first end of the flow tube for enabling a flow of fluid from the flow tube to an internal fluid passage in the valve plate that intersects the bore in the valve plate.

K. A machine as set forth in clause A or any other clause depending from clause A, wherein the valve plate includes a planar surface into which a port opening is integrated.

L. A machine as set forth in clause K or any other clause depending from clause K, wherein housings for setting pistons are attachable to the planar surface of the valve plate.

M. A machine as set forth in clause A or any other clause depending from clause A, wherein a plurality of interconnecting grooves form a balancing region on a surface of the valve plate to enable the valve plate to move directly over a guide path formed by the housing with reduced friction.

N. A machine as set forth in clause A or any other clause depending from clause A, comprising a sensor mounted to monitor an angular position of the flow tube about the pivot axis relative to the housing.

O. A machine as set forth in clause N or any other clause depending from clause N, where the sensor includes a rotatable portion connected to the second end of the flow tube and a fixed portion that is fixed relative to the housing, the sensor providing an output indicative of the relative position of the rotatable portion relative to the fixed portion.

P. A variable displacement hydraulic axial piston machine, comprising a housing; a rotatable member rotatably-mounted in the housing for rotation about a rotation axis; a barrel coupled to the rotatable member, the barrel including a plurality of piston bores; a plurality of pistons movable in respective bores and coupled to the rotatable member for rotation with the rotatable member; a valve plate coupled to the barrel and constrained in the housing for rotation with the barrel, the barrel and the valve plate being pivotally movable together about a pivot axis transverse the rotation axis for varying the displacement of the pistons in the bores; a working fluid within the housing; wherein the housing, the working fluid, and the valve plate cooperate to define a hydrostatic pad between the valve plate and the housing to decrease friction and wear from the relative movement between the housing and the valve plate.

Q. A machine as set forth in clause P or any other clause depending from clause P, where the valve plate includes a plurality of grooves in a surface that faces a corresponding surface of the housing.

R. A machine as set forth in clause P or any other clause depending from clause P, where the valve plate includes a planar surface that faces a corresponding surface of the barrel.

S. A variable displacement hydraulic axial piston machine, comprising a housing; a rotatable member rotatably-mounted in the housing for rotation about a rotation axis; a barrel coupled to the rotatable member, the barrel including a plurality of piston bores; a plurality of pistons movable in respective bores and coupled to the rotatable member for rotation with the rotatable member; a valve plate coupled to the barrel and constrained in the housing for rotation with the barrel, the barrel and the valve plate being pivotally movable together about a pivot axis transverse the rotation axis for varying the displacement of the pistons in the bores; wherein valve plate includes a planar running surface facing a corresponding surface of the barrel.

T. A machine as set forth in clause S or any other clause depending from clause S, where the valve plate includes one or more housings attached to the planar surface for setting pistons.

Further features of the invention will become apparent from the following detailed description when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a variable displacement axial piston machine provided in accordance with the invention.

FIG. 2 is a perspective view of a hydrostatic bearing surface side of the valve plate from the machine of FIG. 1.

FIG. 3 is a perspective view of a valve side of a valve plate from the machine of FIG. 1.

FIG. 4 is a cross-sectional view of the machine of FIG. 1, orthogonal to the view in FIG. 1, taken through a pair of setting pistons.

FIGS. 5-8 are cross-sectional views of the machine of FIG. 1, orthogonal to the view in FIG. 1, taken through a center of the machine axis, showing the orientation at different displacement angles. FIGS. 7 and 8 show the same displacement angle from different points of perspective.

DETAILED DESCRIPTION

The present invention provides an improved design for a hydrostatic axial piston machine, such as a hydraulic pump or a hydraulic motor, or a machine that can operate as either a pump or a motor. This design is particularly applicable to a variable displacement axial piston machine.

The axial piston machine provided by the invention provides several advantages over prior axial piston machines. For example, the machine includes a valve plate that has a hydrostatic bearing surface facing the machine casing to reduce potential wear and to reduce sliding friction; the valve plate has an easily-machined flat running surface; and the valve plate integrates hydrostatic valve plate support and valve plate timing into the flat running surface of the valve plate.

Additionally, the hydrostatic piston machine includes a flow tube separate from the valve plate. As described in additional detail below, the flow tube and the valve plate are not fixed relative to one another so the flow tube can translate axially relative to the valve plate as the valve plate pivots about a pivot axis. This axial movement helps to make up for any tolerance differences between the parts. Additionally, the flow tube allows a feedback sensor to be connected directly on the pivot axis to more accurately measure the valve plate's angular position about the pivot axis.

Turning now to a more detailed description of an exemplary axial piston machine 20 provided by the invention, with reference initially to FIG. 1, the piston machine 20 includes a housing 22, which also may be referred to as a casing, that defines an outer shell of the machine 20 and helps to contain the hydraulic fluid as it moves between a high pressure and a low pressure within the machine 20. The housing 22 typically includes multiple sections that can be secured together during assembly. The housing 22 has an input port (not shown) and an outlet opening 24 connected to an output port (not shown) for receiving or discharging fluid. When operating as a pump, for example, low pressure fluid is received at the input port and high pressure fluid is discharged through the output port.

The machine 20 also includes a barrel 30 that is mounted in the housing 22 for rotation about a rotation axis 32. The barrel 30 typically is approximately cylindrical. The barrel 30 includes a plurality of circumferentially-spaced piston bores 34, for example nine bores, two of which are shown in FIG. 1. All of the machine's bores shown and described here are typically cylindrical, but are not limited to circular cross-sections. Elements that are described as being received in the bores have a cross-section that corresponds to the cross-section of the bore.

Accordingly, the machine 20 also includes a plurality of pistons 36 movable in respective bores 34. Each bore 34 receives an associated piston 36 for reciprocal motion within the bore 34 as the barrel 30 rotates. Each piston 36 includes a central piston rod 37 between a spherical end 38 and a piston end 39.

The spherical end 38 of each piston 36 is affixed in a known manner in a drive disk 40 or other rotatable member rotatably-mounted in the housing 22 for rotation about a machine axis 46. The barrel 30 is pivotally coupled to the drive disk 40 for rotation about the rotation axis 32. In this embodiment, the drive disk 40 is integrally connected to a machine shaft 42 that extends from the housing 22 to convey rotary motion through the housing 22 to or from the drive disk 40. The drive disk 40 and the machine shaft 42 are supported by several bearings 44 seated in the housing 22 for rotation about the machine axis 46. One or more seals 47 further close the opening in the housing 22 through which the shaft 42 passes.

In FIG. 1, the machine axis 46 is aligned with the rotation axis 32 of the barrel 30. That is not always the case, as explained below. In the illustrated embodiment, the machine shaft 42 is coupled to the barrel 30 via a spring-biased synchronizing shaft 48 that can pivot at its ends yet maintains an angular position of the machine shaft 42 relative to the barrel 30 and helps to transfer rotary motion between the barrel 30 and the machine shaft 42.

The barrel 30 is mounted between the drive disk 40 and a sliding valve plate 50. Each piston bore 34 in the barrel 30 has a port 52 that opens to an end face 54 of the barrel 30, which lies against the valve plate 50. The barrel 30 is coupled to the valve plate 50 with an alignment pin 56 for rotation relative to the valve plate 50. The barrel's axis of rotation 32 generally extends through the alignment pin 56. To allow for varying displacement, the valve plate 50 is slidably movable in a curved guide path 58 formed on or inside the housing 22.

A centerline of the guide path 58 serves as the pivot axis 60 of the valve plate 50 and is controlled through one or more setting pistons 62 (FIG. 4) mounted to the housing 22 and coupled to the valve plate 50, as described in further detail below.

The pinned connection between the sliding valve plate 50 and the barrel 30 causes the barrel 30 and the valve plate 50 to be pivotally movable together about the pivot axis 60, thereby varying the displacement of the pistons 36 in the bores 34. The pivot axis 60 is transverse the barrel's axis of rotation 32 and generally perpendicular to the rotation axis 32. While the valve plate's movement is constrained by the guide path 58 on the inside surface of the housing 22, and its connections to the barrel 30 and the setting pistons 62, the barrel 30 is rotatable about the rotation axis 32 relative to the valve plate 50.

Referring now to FIGS. 1-3, with FIGS. 2 and 3 more clearly showing the valve plate 50. The valve plate 50 being movable relative to the housing 22 has several additional features that provide advantages. First, an outer surface 70 of the sliding valve plate 50 helps to define a hydrostatic pad that is supported against the housing 22 to reduce potential wear and to reduce sliding friction. The sliding valve plate 50 is constructed such that the force imparted by the barrel 30 is hydrostatically balanced against the guide path 58 of the machine housing 22. An internal surface face 72 of the sliding valve plate 50, the surface that faces the barrel 30, is machined flat to form a planar running surface 74 that provides tight tolerances and mates with the end face 54 of the barrel 30 to smoothly deliver fluid to and from the bores 34 in the barrel 30.

The valve plate 50 cooperates with the housing 22 and the working fluid within the housing 22, to define a hydrostatic bearing between the outer surface, or bearing surface 70, of the valve plate 50 and the housing 22 to decrease friction and wear from the relative movement between the housing 22 and the valve plate 50. To provide this hydrostatic balance, the outer surface 70 of the valve plate 50 that faces the guide surface 58 of the housing 22 includes a plurality of interconnecting grooves 75 that define a balancing region which is fed with fluid from a high pressure fluid passage (not shown).

On another surface, the valve plate 50 includes a planar operating surface 72 that faces the end face 54 of the barrel 30. The planar surface 54 on the end of the barrel 30 facing the valve plate 50 generally is machined flat. The valve plate 50 also has a running surface 74 that rides against the flat surface 54 of the barrel 30 also generally is machined flat. The sliding valve plate 50 is coupled to the housing via setting pistons 62 with cylindrical housings 78 that are secured to the valve plate 50. The sliding valve plate 50 is constructed such that the generally cylindrical housings 78 for the setting pistons 62 may be made separately from the valve plate 50. This makes it easier to flatten the planar operating surface 72, and more particularly the running surface 74, and then mount the setting piston housings 78.

The running surface 74 may be easily finished via grinding or lapping to achieve a very flat surface for the end face 54 of the barrel 30 to run against. It is much easier to finish the running surface 74 before the setting piston housings 78 are installed. Thus, the setting piston housings 78 are attached to the valve plate 50 after the machining of the flat running surface 74. In one embodiment, two threaded holes are provided in the operating surface 72 and the setting piston housings 78 are threadedly received in the threaded holes.

As shown in FIG. 4, the setting piston's pistons 80 are received in the piston housings 78 and mounted at their ends to the machine housing 22. By selectively introducing high pressure fluid into the setting piston housings 78, the position of the valve plate 50 along the curved guide path 58 can be controlled.

In FIG. 4, the pivot axis 60 (FIG. 1) extends perpendicular to the page, and a setting piston 62 on the left side is fully collapsed, while the right setting piston's piston 80 is fully extended in the respective housing 78 to push the sliding valve plate 50 to the left. The pivoting movement of the valve plate 50 and the barrel 30 is further illustrated in FIGS. 5-8.

Returning to FIGS. 1-3, on the other side of the valve plate 50, the flat running surface 74 includes integral valving, with established valve timing, to eliminate the need for a separate valve plate to open and the bore ports 52 in the barrel 30. In the illustrated embodiment, this valving arrangement includes a semi-circular groove 79 adjacent, and radially inside, the flat running surface 74. Within the groove 79 is a port opening to an internal high pressure fluid passage (not shown). As the barrel 30 rotates, bore ports 52 in the end face 54 of the barrel 30 will be sequentially aligned with this groove or slot 79 to provide a fluid connection between the bore 34 and the fluid passage in the valve plate 50. The fluid passage through the valve plate 50 has an opening in the groove 79 facing the barrel 30, and is connected to the flow tube 90. This passage typically is a high-pressure passage. Thus during the relative rotation between the barrel 30 and the valve plate 50 the outlet ports 52 of the bores 34 are sequentially aligned with the valving in the sliding valve plate 50 to form a fluid connection between the bore 34 and an internal fluid passage in the valve plate 50.

In a pumping operation, as the barrel 30 rotates, each piston bore port 52 sequentially traverses an opening to a supply of low-pressure fluid and low pressure fluid is drawn into the piston bore 34. The valve plate 50 does not rotate with rotation of the barrel 30 about the barrel's rotation axis 32. In the illustrated embodiment, the housing 22 defines a cavity 84 around the barrel 30 that provides the low-pressure supply. When the piston bore port 52 traverses the groove 79 in the valve plate 50, fluid is expelled from the bore 34 at an increased pressure into the high pressure passage in the valve plate 50. Alternatively, when operating as a motor, fluid pressure supplied to the bores 34 when the respective bore port 52 is aligned with the fluid passage in the valve plate 50 causes reciprocal movement of the piston 36. The reciprocal movement of the pistons 36 causes rotation of the drive disk 40 and thus rotation of the shaft 42 of the piston machine 20.

The flow tube 90 defines a conduit for fluid to flow from an outlet opening of the fluid passage in the valve plate 50 through the outlet opening 24 in the housing 22. The flow tube 90 is formed as a separate structure from the valve plate 50, and is connected to the valve plate 50 such that during movement of the valve plate 50 relative to the housing 22 the flow tube 90 may move axially relative to the valve plate 50. The flow tube 90 essentially includes three sections, either of a single composite element or as three separate elements connected together. Thus the flow tube 90 includes a bent tube section 92 interposed between first and second end sections 94 and 96. In this embodiment, the bent tube section 92 bends over 90 degrees and the end sections 94 and 96 are essentially straight. Other angles are possible within the scope of this invention.

The flow tube 90 has a first connector 98 at the first end section 94 that connects the flow tube 90 to the valve plate 50. The connector 98 allows for axial movement of the flow tube 90 relative to the valve plate 50 in a direction parallel to the rotational axis 32 of the barrel 30. Motion parallel to the rotational axis 32 of the barrel 30 includes motion substantially parallel to the rotational axis 32. The flow tube 90 has a second connector 100 at the second end section 96 that connects the flow tube 90 to the outlet opening 24 in the housing 22 while allowing for angular movement of the flow tube 90 about the pivot axis 60.

This construction of the flow tube 90 provides a number of advantages, including the decreased need for tight tolerances, since the first and second sections 94 and 96 of the flow tube 90 can move axially, parallel to the barrel's rotation axis 32, relative to the valve plate 50, as well as axially, parallel to the pivot axis 60, relative to the outlet opening 24 in the housing 22. Thus as the barrel 30 and the sliding valve plate 50 pivot about the pivot axis 60 during changes in displacement, the flow tube 90 can move relative to the valve plate 50 and the housing 22 while both maintaining a fluid connection and accommodating any tolerance differences between the flow tube 90 and the valve plate 50.

A connection block 110 is rigidly attached to an outer surface of the machine housing 22 adjacent the outlet opening 24 in the housing 22. A bore 112 is disposed in the connection block 110 for receiving the connector 100 at the second end of the flow tube 90. The second connector 100 connects the flow tube 90 to not only direct fluid through the outlet opening 24 in the housing 22, but to a fluid passage (not shown) in the bore 112 in the connection block 110 beyond the outlet opening 24 in the housing 22. In other words, although the flow tube 90 is described as providing for fluid flow between the outlet opening in the valve plate 50 and the outlet opening 24 in the housing 22, the flow tube 90 can extend beyond the outlet opening in the valve plate 50 and into the bore

A center of the bore 112 defines a pivot axis for the flow tube 90. Ideally the center of the bore 112, typically the flow tube pivot axis, will be located on a center line that passes through the center of the drive disk 40. The bore 112 in the connection block 110 likely is not perfectly aligned with the pivot axis 60 of the valve plate 50 due to manufacturing processes. In most situations, the pivot axis for the flow tube 90, defined by the bore 112 in the connection block 110, can be considered to be coaxial with the pivot axis 60 for the valve plate 50, defined by the curved guide surface 58 on the inside surface of the housing 22. Generally the pivot axis 60 is parallel to if not aligned with the center line through the drive disk 40. To allow for free movement of the valve plate 50 despite the inherent misalignment of flow tube pivot axis and valve plate pivot axis 60, the flow tube 90 is slidably attached to the valve plate 50. Thus the second connector 100 of the flow tube 90 includes the second end section 96 that is received in the bore 112 in the connection block 110 such that the second end of the flow tube 90 is parallel to the pivot axis 60.

In the illustrated embodiment, the first connector 98 of the flow tube 90 includes the first end section 94 of the flow tube 90 that is received in a bore 114 in the valve plate 50 with a slip fit. Alternatively, the first connector 98 of the flow tube 90 can include the first end section 94 that is received in or telescopes over an extension (not shown) of the passage in the valve plate 50 with a slip fit to permit axial movement while maintaining a fluid connection. Such an extension would extend from the valve plate 50 in a direction parallel to the rotation axis of the barrel 30. In the illustrated embodiment, the passage in the valve plate 50 opens into the bore 114, and the bore 114 extends parallel to the rotation axis of the barrel 30. The bore 114 or the end of the flow tube 90 contain seals 116 to prevent fluid leakage at the connection of the flow tube 90 and the valve plate 50 and the connection block 110. Therefore, as the sliding valve plate 50 pivots due a force imparted by the setting pistons 60 to change the displacement of the valve plate 50 in the housing 22, the flow tube 90 pivots in the bore 112 of the connection block 110 and also may translate a short distance axially in the bore of the valve plate 50 or the bore in the connection block 110, as needed, to accommodate manufacturing dimensional variations.

This configuration of the flow tube 90 and the valve plate 50 on one end and the flow tube 90 and the connection block 110 on the other end allows the flow tube 90 to move in two orthogonal directions without compromising the conduit for fluid flow. This configuration also ensures that the valve plate 50 puts little or no load on the flow tube 90 as the valve plate 50 moves relative to the housing. This improves performance and reliability by reducing the bearing loads and sliding contact between the mating parts. For fast and accurate controlled displacement, the valve plate 50 and the flow tube 90 can move freely relative to each other and the flow tube 90 can move freely relative to the connection block 110.

An additional benefit of the flow tube 90 interface with the connection block 110 is that a rotary sensor 120 can be attached to the flow tube 90 on the pivot axis 60, which increases the ability to approximate the actual displacement angle of the valve plate 50. The second connector 96 in the second end section 96 of the flow tube 90 is notched to receive a rotatable portion of the rotary sensor 120. The remainder of the rotary sensor 120 is attached to the connection block 110. As a result the rotatable portion of the sensor 120 rotates relative to the remainder of the rotary sensor 120 during pivoting movement of the flow tube 90. The displacement of these components relative to each other can be used to monitor the angular position of the valve plate 50 within the housing 22. The sensor 120 can output an electrical signal representing the angular position of the flow tube 90. The illustrated sensor 120 provides an output via a wire 122, but other outputs are possible. For example, a flag member or other indicator that extends outside the housing 22 could be used to mirror the angular position of the flow tube 90 to provide a visual indication of the flow tube's position.

In the illustrated configuration a single flow tube 90 is associated with the valve plate 50, while the other fluid conduit is formed by the barrel 30 and the machine housing 22. In this configuration, only the conduit formed by the flow tube 90 may be exposed to high pressures, for example 400 bar, while the conduit formed by the housing 22 may only be exposed to relatively low pressures, 5 bar. Alternatively, two or more flow tubes may be utilized to allow high pressure in two conduits or high pressure in one and low pressure in the other.

Thus, the present invention provides a variable displacement hydraulic axial piston machine 20 that comprises: (a) a housing 22 having an outlet opening 24; (b) a barrel 30 mounted in the housing 22 for rotation about a rotation axis 32; (c) a plurality of pistons 36 movable in respective bores 34 in the barrel 30; (d) a valve plate 50 coupled to the barrel 30 and having a fluid passage therein that ends at an outlet opening; and (e) a flow tube 90 connecting the outlet opening in the valve plate 50 to the outlet opening 24 in the housing 22. The barrel 30 and the valve plate 50 are pivotally movable together about a pivot axis 60 transverse the rotation axis 32 for varying the displacement of the pistons 36 in the bores 34. An end of the flow tube 90 connects to the valve plate 50 while allowing for axial movement of the flow tube 90 relative to the valve plate 50 in a direction parallel to the rotational axis 32 of the barrel 30.

Although the invention has been shown and described with respect to a certain preferred embodiment, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. 

1. A variable displacement hydraulic axial piston machine, comprising: a housing having an outlet opening; a rotatable member rotatably-mounted in the housing for rotation about an axis; a valve plate having a fluid passage therein that ends at an outlet opening, a barrel mounted between the valve plate and the rotatable member, the barrel being coupled to the valve plate for rotation relative to the valve plate and the barrel being pivotally coupled to the rotatable member for rotation about a rotation axis, the barrel including a plurality of piston bores; a plurality of pistons movable in respective bores in the barrel and coupled to the rotatable member for rotation with the rotatable member, the barrel and the valve plate being pivotally movable together about a pivot axis transverse the rotation axis for varying the displacement of the pistons in the bores; a flow tube connecting the outlet opening of the valve plate to the outlet opening in the housing, the flow tube having a first connector at a first end that connects the flow tube to the valve plate in a direction parallel to the rotational axis of the barrel while allowing for axial movement of the flow tube relative to the valve plate in a direction parallel to the rotational axis of the barrel, and a second end opposite the first end, the flow tube having a second connector at the second end that connects the flow tube to the outlet opening in the housing for axial movement in a direction coaxial with the pivot axis while allowing for angular movement of the flow tube about the pivot axis.
 2. A machine as set forth in claim 1, where the first connector includes an end section of the flow tube that is received in a bore in the valve plate with a slip fit.
 3. A machine as set forth in claim 1, where the second end of the flow tube is parallel to the pivot axis.
 4. A machine as set forth in claim 1, where the barrel includes a plurality of circumferentially-spaced cylindrical bores.
 5. A machine as set forth in claim 1, where the valve plate is movable relative to the housing.
 6. A machine as set forth in claim 1, where the flow tube is formed as a separate structure from the valve plate.
 7. A machine as set forth in claim 1, where the first end of the flow tube is movable axially within the bore of the valve plate.
 8. A machine as set forth in claim 1, where the second end of the flow tube is received in a bore that is coaxial with the pivot axis.
 9. A machine as set forth in claim 1, where the second end of the flow tube is received in a bore with a slip fit for rotation about the pivot axis.
 10. A machine as set forth in claim 1, wherein a radial opening extends through the first end of the flow tube for enabling a flow of fluid from the flow tube to an internal fluid passage in the valve plate that intersects the bore in the valve plate.
 11. A machine as set forth in claim 1, wherein the valve plate includes a planar surface into which a port opening is integrated.
 12. A machine as set forth in claim 11, wherein housings for setting pistons are attachable to the planar surface of the valve plate.
 13. A machine as set forth in claim 1, wherein a plurality of interconnecting grooves form a balancing region on a surface of the valve plate to enable the valve plate to move directly over a guide path formed by the housing with reduced friction.
 14. A machine as set forth in claim 1, comprising a sensor mounted to monitor an angular position of the flow tube about the pivot axis relative to the housing.
 15. A machine as set forth in claim 14, where the sensor includes a rotatable portion connected to the second end of the flow tube and a fixed portion that is fixed relative to the housing, the sensor providing an output indicative of the relative position of the rotatable portion relative to the fixed portion.
 16. A variable displacement hydraulic axial piston machine, comprising: a housing; a rotatable member rotatably-mounted in the housing for rotation about a rotation axis; a barrel coupled to the rotatable member, the barrel including a plurality of piston bores; a plurality of pistons movable in respective bores and coupled to the rotatable member for rotation with the rotatable member; a valve plate coupled to the barrel and constrained in the housing for rotation with the barrel, the barrel and the valve plate being pivotally movable together about a pivot axis transverse the rotation axis for varying the displacement of the pistons in the bores; a working fluid within the housing; wherein the housing, the working fluid, and the valve plate cooperate to define a hydrostatic pad between the valve plate and the housing to decrease friction and wear from the relative movement between the housing and the valve plate.
 17. A machine as set forth in claim 16, where the valve plate includes a plurality of grooves in a surface that faces a corresponding surface of the housing.
 18. A machine as set forth in claim 16, where the valve plate includes a planar surface that faces a corresponding surface of the barrel.
 19. A variable displacement hydraulic axial piston machine, comprising: a housing; a rotatable member rotatably-mounted in the housing for rotation about a rotation axis; a barrel coupled to the rotatable member, the barrel including a plurality of piston bores; a plurality of pistons movable in respective bores and coupled to the rotatable member for rotation with the rotatable member; a valve plate coupled to the barrel and constrained in the housing for rotation with the barrel, the barrel and the valve plate being pivotally movable together about a pivot axis transverse the rotation axis for varying the displacement of the pistons in the bores; wherein valve plate includes a planar running surface facing a corresponding surface of the barrel.
 20. (canceled)
 21. A variable displacement hydraulic axial piston machine, comprising: a housing having an outlet opening; a rotatable member rotatably-mounted in the housing for rotation about an axis; a valve plate having a fluid passage therein that ends at an outlet opening, a barrel mounted between the valve plate and the rotatable member, the barrel being coupled to the valve plate for rotation relative to the valve plate and the barrel being pivotally coupled to the rotatable member for rotation about a rotation axis, the barrel including a plurality of piston bores; a plurality of pistons movable in respective bores in the barrel and coupled to the rotatable member for rotation with the rotatable member, the barrel and the valve plate being pivotally movable together about a pivot axis transverse the rotation axis for varying the displacement of the pistons in the bores; a flow tube connecting the outlet opening of the valve plate to the outlet opening in the housing; a rotary sensor attached to the flow tube on the pivot axis to monitor the angular position of the valve plate within the housing. 